Plasmas are widely used to modify the surface properties of materials and are now indispensable in etching sub-micron features. These features are created using a mask to define the feature, reactive neutrals (radicals) to attack the unmasked areas chemically, and energetic ions to remove the debris and provide directionally. The plasma provides both the ions and radicals. In conventional etchers the ions are almost always positive and are accelerated onto the materials by an electric field. Because most materials being etched are poor conductors, a negative current must accompany the positive ion current, to avoid charging the surface. The simplest solution is to apply rf fields that drive positive ions into the material during one part of the rf cycle and negatively charged particles during the other part. The rf frequency most commonly used is 13.56 MHz.
Conventional etchers use electromagnetic fields to heat plasma electrons to ionize a background gas, and the plasmas thus formed necessarily contain large numbers of free electrons. In electronegative gases, some of the electrons attach to the molecules to form negative ions, but the electrons continue to carry most of the negative rf current because the ions are much heavier and less mobile. Moreover, the electrons generate an electrostatic field that prevents negative ions from leaving the plasma. The positive ions and free radicals then do the actual etching of reactive material in contact with the plasma, while the electrons neutralize any bulk charge left on the surface of the material. Negative ions, while often present, are unsued in conventional etchers.
Using electrons to neutralized positive surface charge works well for large-scale features but not small-scale features. This is because the light and hot electrons flow in all directions, whereas the cold and massive ions are driven directly toward the material by the applied and self-fields. The ions therefore preferentially strike the bottom of a deep narrow trench, whereas spread out and strike the side walls of the trench. The bottom of the trench thus charges positively while the side walls charge negatively, and this difference in charge generates a transverse electrostatic field that deflects ions into the side walls. The side wall then begin to etch and erode, thus deforming the trench. Deep narrow trenches with straight side walls are therefore difficult to form with electron-ion plasmas.
One possible solution is to use negative ions rather than electrons to neutralize the surface charge. This requires an ion-ion plasma consisting mainly of positive and negative ions but few electrons. Unlike electrons, negative ions flow directly into a material when accelerated through a thin, electrostatic sheath adjacent to the material. Moreover, negative ions etch as well as, and possibly better than, positive ions. In ion-ion plasmas, positive ions flow toward the material during one half cycle of the rf field, while negative ions flow during the other half cycle. However, the rf frequency must now be reduced to 1 MHz or less, to give the massive ions time to respond to the fields. Also, square rf pulses can be used in place of sinusoidal pulses, to reduce the energy spread of the ions and thereby improve etch selectively in different materials. Since both current carriers are now directed toward the material, deeper and narrower channels can be formed using ion-ion plasmas. The aspect ratio ultimately achievable is then limited by chemical etching from the isotropic radicals alone. This limit, which has yet to be reached in present-day etchers, is approached with ion-ion plasmas provided the ions are cold and traverse the rf sheath while suffering few collisions.
Conventional electromagnetic discharge sources use hot electrons to generate a discharge and thus naturally generate electron-ion plasmas. These sources include capacitively coupled discharges, inductively coupled discharges, helicons, surface waves, and electron-cyclotron-resonance reactors. However, if the electromagnetic heating fields are turned off, the plasma will convert into an ion-ion plasma in many of the halogen-based gases commonly used for etching. This is because, the dissociative attachment rate rises, in these gases, as the electrons temperature drops, and thus the electrons attach during the afterglow (“off” phase) to form negative ions. Pulsing any conventional source can thus produce an ion-ion plasma late in the afterglow. When the heating fields are on, the electrons are hot and produce an electron-ion plasma. When the heating fields are off, the electrons cool, the plasma decays, and an ion-ion plasma eventually forms. However, because the electrons are hotter and more mobile than the ions, this conversion typically occurs only late in the afterglow when the electron density has fallen to several orders of magnitude below the ion density. Only at that point are negative ions able to leave the plasma.
The Charged Particle Physics Branch (Code 6750) at the Naval Research Laboratory has developed a plasma source for etching called the Large Area Plasma Processing System (LAPPS). This system is the subject of U.S. Pat. Nos. 5,182,496 and 5,874,807, both of which are incorporated herein by reference, in their entireties. This plasma source uses a magnetically confined, sheet electron beam to ionize a background gas and produce a planar electron/ion plasma. Electron beams exhibit high ionization and dissociation efficiency of the background gas. In addition, the plasma production process is largely independent of the ionization energies of the gas or the reactor geometry. Since the plasma volume is limited only by beam dimensions, the usable surface area of the plasma thus can exceed that of other plasma sources.
Although pulsing a conventional plasma source can produce ion-ion plasmas, the technique suffers from several serious limitations. One limitation is that hot electrons drive the ion flux during the electron-ion phase, whereas cold ions drive the ion flux during the ion-ion phase. As a result, the ion flux during the electron-ion phase is orders of magnitude larger than the ion flux during the ion-ion phase. In addition, the ion-ion phase persist for only a brief portion of the afterglow and therefore for an even shorter portion of the total period. The net result is that most of the etching occurs during the electron-ion phase rather than during the ion-ion phase. The useful duty cycle and efficiency of ion-ion etching from conventional, pulsed sources is thus low. Nevertheless, despite these limitations, pulsed plasmas have been shown to improve etch quality.
Therefore, it would be desirable to produce an ion-ion plasma with a high degree of control that is continuous in time.